12 SOME MINUTE ANIMAL PARASITES 



in the case of those parasitic Protozoa that cause the 

 death of cattle or man. It is for the scientist to 

 investigate carefully for the weak spot in order that 

 the veterinarian, stock-breeder, or poultry farmer 

 may apply the remedy, assuming that preventive 

 measures have failed. 



The grouping or classification of any set of living 

 animals is always a matter of some difficulty, for 

 with advancing knowledge alterations are bound to 

 occur. Classification is but tentative, and marks the 

 state of our knowledge for the time being. In this 

 book only the broadest possible classification will be 

 given. The life-histories of the organisms are the 

 epitomes of interest, not the " pigeon-hole " artificial 

 arrangements that the world calls classification 

 tables. 



Among the lowliest of the Protozoa are the 

 organisms, often almost undifferentiated nucleated 

 masses of protoplasm, whose method of locomotion 

 is restricted to an outflowing of the protoplasm in 

 one direction, accompanied by the withdrawal of the 

 living substance from another part. The protruded 

 masses are known as pseudopodia, and it is by 

 their agency that the progression of the organism 

 is accomplished. Little differentiation is exhibited 

 by many of the Protozoa of this first group. They 

 have no cell mouth, no cell anus, no food canal, no 

 specially formed locomotor apparatus. They are 

 slow, relatively passive, and pass their time either 

 motionless or creeping slowly over the pond bottom, 

 or, in the parasitic members, over the intestine of 

 their hosts and even invade the cells of the latter for 



